There have been several methods for manufacturing sintered porous metal bodies. Among them a casting method, a foaming method, a burning synthetic method and a powder sintering method have been known. One of powder sintering methods is a spacer method in which a spacer material for forming spaces in a sintered body and metal powder as a base material are mixed, molded and sintered to thereby produce a porous body.
Patent document No. 1 and non-patent document No. 1 disclose aluminum based porous materials. In patent document No. 1, there is disclosed a method of manufacturing a sintered porous metal body, which has excellent impact absorption, featured by preparing a mixture of a powder of aluminum or aluminum alloy and a water soluble spacer material powder, charging the mixture into a vessel, applying a pulsating current to the mixture powder under a compression pressure to sinter the aluminum or aluminum alloy mixture, and dissolving the spacer out from the sintered body with water to thereby obtain a sintered porous body. Further, in patent document No. 1, aluminum powder having a particle size of 3 μm and NaCl powder as a spacer material, having a particle size of 200 to 300 μm are mixed, and the resulting mixture is sintered by applying a pulsating current at 480° C. for 5 minutes under a compression pressure of 20 MPa in a graphite mold to thereby produce a porous aluminum body.
In non-patent document No. 1, aluminum powder having particle size of 450 μm and NaCl powder having a particle size of 300 to 1000 μm as a spacer material are mixed and the mixture is sintered in a steel mold at 680° C. for 180 minutes after molding the mixture under a compression pressure of 200 MPa to produce a porous aluminum body.
Non-patent document No. 2 discloses that though it has been a common knowledge that microwave heating is not useful for heating metal materials because the microwave heating uses dielectric loss of dielectric materials. Heating and sintering of metal powders by the microwave are performed by induction loss or magneto-loss due to a skin effect.
In general, it is said that sintering of aluminum powder is extremely difficult because native oxide (alumina) formed on the surface of the powder is thermally and chemically stable very much. Normally, the native oxide film formed on the metal powder may be reduced and removed by sintering it in a reducing atmosphere, but aluminum oxide or magnesium oxide is not reduced because the oxides have low standard thermo-dynamic quantity.
Non-patent document No. 1 discloses a method for molding powder under a pressure as high as 200 MPa. It is assumed that the native oxide film is destroyed under a shearing force by elastic-deforming the aluminum powder to thereby bring aluminum powder into contact with each other without the native oxide film and accumulate strain energy therein. After a long sintering time, the aluminum powder diffuses each other releasing the strain energy to barely case the powder to be sintered.
However, in case of non-patent document No. 1, aluminum powder (particularly, pure aluminum powder) may enter into gaps of the mold at the time of high pressure molding because of its low hardness and softness, which causes galling or damage to the mold. Since generally employed heating with heaters is conducted in an atmosphere, the article to be heated and the atmosphere as well as furnaces must be heated, which needs a long time for sintering. As a result, crystalline grains in the aluminum powder grow coarse in size to lessen mechanical strength.
Patent document No. 2 discloses a sintering method in which a material to be sintered selected from the group consisting of ceramics, ceramic composite materials and metallic materials is covered with a layer of granular susceptor, a protecting gas is introduced around the material, and microwave energy is irradiated to the material and the susceptor, wherein the susceptor layer comprises (a) a dominant amount of microwave susceptor material and (b) a small amount of heat resisting mold-separating agent, which is dispersed in the susceptor material or is supplied as a coating on the susceptor.
Patent document No. 3 discloses a composite body in which metal is impregnated into a porous ceramic body, the entire surface of the composite body being covered with a layer of the metal; the porous ceramic body is at least one member selected from the group consisting of silicon carbide, aluminum nitride, silicon nitride, alumina and silica; the metal is aluminum or magnesium; and a porosity of the porous ceramics is porous silicon carbide having porosity of 20 to 50% and the metal is aluminum.
Patent document No. 4 discloses a method of manufacturing a light metal composite body which comprises forming a molding article of porous metal body having a metal alloy layer on the surface thereof wherein silicon carbide particles are dispersed, placing the molded article in a mold, and casting the molded article with molten aluminum alloy.
The pulsating current sintering method comprises filing a mixed powder of aluminum (Al) and sodium chloride (NaCl) in a graphite mold, and heating the mixture with pulsating current while the mixture is compressed in a uni-axial direction to sinter the mixture. Generally, it is said that the pulsating current sintering method can heat the sample effectively to sinter it within a very short period of time.
However, dispersion or fluctuation of characteristics is the problem which is caused by temperature distribution of the sample or carbon mold at the time of sintering so that it is very difficult to obtain a uniform temperature distribution. In addition, the pulsating current sintering method has low productivity because a number of samples are not sintered at one time, which is performed by heating with the conventional heaters, and a size of the samples is limited to a size of the graphite mold, which makes scale-up of the samples difficult.
On the other hand, it is said that when microwave is used, a quick heating, an inner heating or quick sintering is possible by virtue of induction loss or magnetic loss due to skin effect of metal powder. However, if a molding pressure or density is high, molded articles are not an agglomerate of individual powders, but a bulk body in which the individual powders are mechanically bonded.
Microwave is mainly reflected in the surface of the bulk body, part of which heats the surface and its vicinity of the bulk body by virtue of skin effect, but amount of heat generation is small and sintering does not occur. Further, in microwave sintering, it is necessary to increase the compression pressure or density of the molding in order to perform sintering by mutual diffusion while the strain energy of the powders in metal contact with each other is released.
FIG. 1 shows a sintering density of sintered articles of pure aluminum powder that were produced by sintering molded bodies with microwave and heater at 645° C. each having a different density. In this case, pure aluminum powder was not mixed with other materials such as insulating powder (sodium chloride), dielectric powder (silicon carbide) or semiconductor powder (carbon). In this figure, the higher the sintering density, the larger the volume shrinkage by sintering the sintered articles exhibit. In addition, □, Δ and ◯ represent the articles produced by heating with heaters and ▪, ▴ and ● represent the sintered articles produced by heating with microwave. In case of heating with the heaters, volume shrinkage was not observed even after heating for 60 minutes. This was because the native oxide film hindered sintering.
On the other hand, in case of microwave heating, a volume shrinkage was observed within 10 to 30 minutes, and a remarkable volume shrinkage was observed particularly in molded articles having a molding density of about 70%. This was because if there were gaps in the molded articles, microwave could permeate into the interior of the molded articles so that each powder is heated by virtue of skin effect. That is, since the native oxide layer and its vicinity were preferentially heated than the interior of the powder to assist diffusion between the powder particles.
Accordingly, it is important to control density of molded articles so as to let the microwave permeate into the interior of the molded articles in heating the molded articles of the metal powder.
In powder metallurgy, net-shape or near net-shape is an important feature. Normally, the molding density is 90% or more, and such molded articles cannot be sintered by microwave heating. Thus, in non-patent document No. 2, a susceptor made of SiC, which absorbs microwave to induce heating, is arranged around the articles to effect indirect heating.
Further, since the microwave heating is caused by spontaneous heat generation of the molded articles, an amount of heat depends on shapes of powder or size. In addition, it is said that to secure a constant temperature distribution is extremely difficult because microwave electromagnetic field tends to concentrate at corners of the articles, and the corners are excessively heated than other portions. Moreover, temperature distribution in the interior is very complicated.